Missing panels in Papers library view

In case you have trouble viewing your Papers library correctly,
for example, if the left-hand side library panel is missing, try
removing the Papers2 preference file
Library/Preferences/com.mekentosj.papers2.plist from under your
home directory. Note that when you then open Papers2 again, you
might have to connect it to the location of your library the next
time you start it up.

Note that if you use OS 10.7 Lion or above, to access your
Library folder, you will need in the Finder to use the Go menu
while pressing the option (alt) key, which will add a 'Library'
item to the list.